Process for the production of aromatic polycarboxylates

ABSTRACT

IMPROVED CATALYTIC DISPROPORTIONATION OF AROMATIC CARBOXYLATES TO AROMATIC POLYCARBOXYLATES CONTAINING AT LEAST ONE ADDITIONAL CARBOXYL GROUP IS ACHIEVED BY CARRYING OUT THE CATALYTIC DISPROPORTIONATION PROCESS IN THE PRESENCE OF A CONVENTIONAL DISPROPORTIONATION CATALYST AND A MELAMINE ADJUVANT OF THE FORMULA   2,4,6-TRI((R-)2-N-)-S-TRIAZINE   WHEREIN R IS HYDROGEN OR AN ALKYL RADICAL HAVING FROM 1 TO 4 CARBON ATOMS THEREIN.

United States Patent O US. Cl. 260-515 P 6 Claims ABSTRACT OF THEDISCLOSURE Improved catalytic disproportionation of aromaticcarboxylates to aromatic polycarboxylates containing at least oneadditional carboxyl group is achieved by carrying out the catalyticdisproportionation process in the presence of a conventionaldisproportionation catalyst and a melamine adjuvant of the formulawherein R is hydrogen or an alkyl radical having from 1 to 4 carbonatoms therein.

BACKGROUND OF THE INVENTION It is known that alkali metal salts ofvarious carboxylic acids having one or more carboxyl groups attached toan aromatic ring system can be converted into salts of the correspondingcarboxylic acids having at least one additional carboxyl group in themolecule by thermal conversion or catalytic disproportionation atelevated temperatures and pressures. While such a process has provenindnstrially attractive for the production of various types ofpolycarboxylic acids such as terephthalic acid, the process asheretofore practiced suffers from the disadvantage exhibiting relativelylow efficiency. In addition, the handling of the high pressure systemsto obtain conversion requires the use of expensive high pressureequipment.

It is thus an object of the present invention to provide an improvedprocess for the catalytic formation of polycarboxylic acids.

Another object of the present invention is to provide a process for theformation of alkali metal salts of aromatic polycarboxylic acids.

Another object is to provide an improved catalytic disproportionationsystem for formation of salts of polycarboxylic acids.

Other aspects, objects and the several advantages of this invention willbe apparent from the following specification and claims.

In accordance with this invention, we have discovered that in theprocess for the formation of aromatic polycarboxylates bydisproportionation of an alkali metal salt of the correspondingcarboxylic acids, containing at least one less carboxyl group, anefficient system is achieved, giving good conversion to the desiredalkali metal salts of said polycarboxylic acids which is operable atpressures lower than heretofore generally possible by carrying out thedisproportionation reaction in the presence of a conventionaldisproportionation catalyst and an adjuvant compound of the formula andwherein R is hydrogen or an alkyl radical having from 1 to 4 carbonatoms therein.

Thus, by the present invention the conversion of an alkali metalaromatic carboxylate to the corresponding aromatic polycarboxylatehaving at least one additional carboxylate group which can besubsequently converted to the corresponding aromatic polycarboxylic acidis readily carried out in an efiicient system when one or more adjuvantcompounds of the formula as above defined are utilized as a catalystadjuvant.

The disproportionation process of this invention for potassium benzoatecan be represented as follows:

Catalyst Adjuvant As starting materials for the process of thisinvention, in addition to the presently preferred alkali metal salts ofbenzoic acid, the salts of other aromatic monoor polycarboxylic acidscan be used. Such salts are those of the formula i Ar-(C-OM)n wherein Aris an aromatic or alkyl aromatic group having 6 to 15 carbon atomstherein, M is an alkali metal, and n is 1, 2 or 3.

In carrying out the conversion of compounds of the formula Ar(COOM)advantageously, the alkali metal salts, preferably the potassium saltsor the sodium salts, are used. The lithium, rubidium and cesium saltscan also be used for this reaction. In the place of the salts, mixturescan also be used which are transformed into the salts upon heating, forexample, mixtures of carboxylic acid, anhydrides, and alkali metalcarbonates. Mixtures of salts of two different metals, for example,mixtures of the sodium and potassium salts, can be used.

Such salts are readily prepared from the corresponding acids. Such acidsinclude, for example, benzoic acid, 2- naphthalene carboxylic acid,4-biphenyl carboxylic acid, 2,6-naphthalene dicarboxylic acid, phthalicacid, isophthalic acid, terephthalic acid, 2-anthracenecarboxylic acid,1,S-anthracenedicarboxylic acid, 1,5,9-anthracenetricarboxylic acid,3-phenanthrenecarboxylic acid, 1,4,9-phenanthrenetricarboxylic acid,2,3,4-trimethylbenzenecarboxylic acid,2,4,6-trimethylbenzene-1,3-dicarboxylic acid, 2-ethyl-4-hexyl-6-methylbenzene-l,3-dicarboxylic acid, 2,4-dibutylbenzene-l,3,5-tricarboxylic acid, and the like. Also, mixtures ofsuch acids may be used. In all of these carboxylic acids, the aromaticring may carry alkyl radicals in addition to the carboxyl groups. Ifaromatic monocarboxylic acid salts are used as starting materials forthe performance of the process according to the invention, the reactionproducts obtained thereby are industrially valuable dicarboxylic aciddisalts and in many cases are those dicarboxylic acid disalts which havea symmetric structure,

for example, terephthalic acid disalt, naphthalene-2,6-dicarboxylic aciddisalt, and the like.

The aromatic polycarboxylates which are produced according to theprocess of this invention can be recovered by any means known to theart. It is also within the scope of this invention to convert sucharomatic polycarboxylates into the respective aromatic polycarboxylicacids and subsequently recover such acids by any means known to the art.

For example, the reaction product is first dissolved in water.Subsequently, the salts formed by the reaction can be transformed intothe corresponding free acids by acidifying the solution with organic orinorganic acids or by introducing carbon dioxide into the solution atatmospheric or elevated pressure, and then separating the free acidsfrom the acidified solution. The individual reaction products may beseparated from each other and isolated in pure form by methods which arebased upon their different solubilities or volatilities and maythereafter, if desired, be transformed into their derivatives. The saltmixture produced by the reaction may also be transformed directly intoderivatives of the acids, for example, into their esters or halides, andthese derivatives may be purified, if desired, by fractionaldistillation.

The resulting polycarboyxlic acids which are ultimately produced fromthe salts which are produced through use of the process of thisinvention are well known to the art and are useful for a variety ofpurposes.

For example, the process can be used to convert potassium benzoate topotassium terephthalate which is readily converted to terephthalic acid.Terephthalic acid has at least three major applications at present.Perhaps the most important of such applications is in the manufacture ofsynthetic fibers of the polyester type. Also of considerable importanceis the use of terephthalic acid as an intermediate for the preparationof polyester film. Considerable quantities of terephthalic acid are alsoemployed in the manufacture of terephthalic acid based plasticizers.

The process of this invention can be effected in a batchwise orcontinuous manner. Conventional equipment can be employed.

The process of this invention constitutes a further valuable improvementover prior art methods in that high pressures do not need to be employedto effect substantial yields. The resultant savings in equipment outlayand compression costs constitute a significant economlc advantage.

It is essential to the process of this invention that at least oneadjuvant compound of the formula (as defined above) be employed. Suchadjuvants are generally employed in the range of about 0.001 to 2 molsper mol of alkali metal aromatic carboxylate, more preferably in therange of about 0.1 to 0.5 mol per mol of aromatic carboxylate.

The melamine adjuvant compounds employed as adjuvant compounds in thepresent invention are known compounds which can be readily prepared bythe process as disclosed by Kaiser et al., J. Am. Chem. Soc. 73, 29846(1951).

Examples of suitable melamine adjuvants are melamine, dimethylmelamine,diethylmelamine, dipropylmelamine, dibutylmelamine and the like.

It is also essential to the process of this invention that a suitablecatalyst be employed in the reaction medium. Generally, in the range ofabout 0.1 to 100 g. of catalyst per mol of alkali metal aromaticcarboxylate are employed, more preferably in the range of l to 50 g. pergram 11101 are employed.

Catalysts which can be employed according to the process of thisinvention include cadmium, zinc, iron, lead and mercury, as well ascompositions which contain these elements. The oxides and salts of thenamed metals, particularly oxides, carbonates and halides of the metalsare preferred. The oxides, halides, and carbonates of zinc and cadmiumare particularly preferred. Some examples of suitable catalyticcompounds include metallic cadmium, cadmium oxide, cadmium iodide,cadmium chloride, cadmium fluoride, cadmium sulfate, cadmium phosphate,cadmium carbonate, cadmium acetate, cadmium soaps, cadmium benzoate,cadmium phthalate, metallic zinc, zinc oxide, zinc iodide, zincchloride, zinc sulphate, zinc phosphate, zinc phthalate, zincisophthalate, and the like.

The conversion of the alkali metal carboxylate is effected substantiallycompletely in the absence of oxygen or water. In general, temperaturesin the range of about 350 to 500 C. are employed, more preferably in therange of 400 to 450 C.

The process is preferably carried out in an atmosphere which is at leastvolume percent carbon dioxide with the remaining portion thereof beingmade up of inert gases which are substantially nonreactive in thereaction system. Examples of such inert gases are nitrogen, methane,argon, neon, butane, ethane and helium. In one presently preferredembodiment of this invention, the process is carried out in anatmosphere which is at least volume percent carbon dioxide with theremaining portion thereof being made up of an inert gas.

Pressures in the range of 0 p.s.i.g. to 5000 p.s.i.g. or more can beemployed but it is advantageous and preferable, in keeping with onepreferred embodiment of the instant invention, that pressures in therange of 0 to 1000, preferably 0 to 10 p.s.i.g., be employed.

Sufficient reaction time should be employed to effect the desired degreeof conversion. Generally reaction times in the range of about .5 minutesto about 48 hours are suitable.

Diluents for the reaction system can be employed as desired and canconstitute up to 80 weight percent of the reaction mixture. Any diluentcan be employed which is substantially nonreactive to the reactionenvironment. Examples of suitable diluents include heptane, benzene,naphthalene, particulate silica, particulate carbon, cyclohexane, andthe like.

Although not required, the conversion can be effected in the presence ofan alkali metal or an alkaline earth metal carbonate. Normally suchcompounds, when utilized, are employed in an amount in the range ofabout 0.1 to g. of carbonate or each gram mol of alkali metal aromaticcarboxylate, preferably in the range of 1 to 50 grams per gram mol. Thepresence of such compounds is conventionally considered to promoteacidbinding in the system. In place of the carbonates the salts of otherweak acids may be used, for example, the bicarbonates, formates, oroxalates.

If the starting materials are solids, they are preferably used in dryand finely divided form and are intimately mixed with each other. Inorder to avoid local overheating and decomposition caused thereby, aswell as to avoid caking of the reaction mixture, it is sometimesadvantageous to maintain the reaction mass in motion. This may beaccomplished, for example, by carrying out the reaction in vesselsprovided with a stirring device, in screw conveyors or in rocker orrotary autoclaves. However, uniform heating may also be effected bydistributing the starting materials in thin layers and in this methodthe reaction mixture may be agitated or may remain stationary. Goodyields, however, are also obtained without these special measuresprovided care is taken that local overheating is avoided. The processmay also be carried out in a fluidized bed of the solid startingmaterials.

The following example will enable persons skilled in the art to betterunderstand and practice the invention and is not intended to belimitative.

In the following example, conversion was calculated by: 1, calculatingthe weight of benzoic acid which was equivalent to the weight ofpotassium benzoate which was converted to any product; 2, calculatingthe weight of benzoic acid which was equivalent to the weight of thepotassium benzoate charged; 3, dividing the weight of benzoic acidcalculated in 1 by the weight of benzoic acid calculated in 2; and 4,multiplying the value calculated in 3 by 100. Efficiency was calculatedby determining the weight of potassium terephthalate formed, convertingthat weight to an equivalent weight of terephthalic acid, dividing thatvalue by the weight of benzoic acid calculated in 1 above, andmultiplying the resultant value by 100. Quantities of carboxylatesand/or polycarboxylates were calculated by converting the carboxylatesand/ or polycarboxylates to the respective methyl esters by reactionwith methanol and sulfuric acid, and subsequently determining thequantities of the respective methyl esters by gas chromatography incomparison with known authentic standards.

EXAMPLE A series of runs were carried out wherein a total of 3.2 g. ofpotassium carbonate, 0.20 g. of cadmium carbonate and various amounts ofmelamine as indicated were charged to a reactor. Following purge of thesystem with carbon dioxide at 27 C. and 0 p.s.i.g. the reactors weresealed and heated to 815 F. 435 C.) for one hour. All handling of thereactants prior to the sealing of the re actors was done in a dry box.

The following results were obtained:

Efficiency, wt. percent Adj uvant, Conversion, benzoic grams wt. percentacid formula Ar(COOM) wherein Ar is an aromatic or alkyl aromatichydrocarbon group having from 615 carbon atoms therein, said alkylaromatic hydrocarbon group having up to three alkyl substituents eachcontaining up to and including 6 carbon atoms, M is an alkali metal andn is 1, 2 or 3 so as to form an aromatic polycarboxylate containing atleast one additional carboxyl group by heating said aromatic carboxylatein an atmosphere containing at least volume percent carbon dioxide andin the presence of a disproportionation catalyst, the improvement whichcomprises carrying out said disproportionation process in the presenceof at least one adjuvant compound of the formula wherein R is hydrogenor an alkyl radical having from 1 to 4 carbon atoms therein.

2. The process of claim 1 wherein said adjuvant is present in an amountin the range of about 0.001 to 2 mols per mol of alkali metal aromaticcarboxylate.

3. The process of claim 1 wherein there is additionally present analkali metal carbonate in an amount in the range of 0.1 to 100 g. ofcarbonate per gram mol of alkali metal aromatic carboxylate.

4. The process of claim 1 wherein there is additionally present analkaline earth carbonate in an amount in the range of 0.1 to 100 g. ofcarbonate per gram mol of alkali metal aromatic carboxylate.

5. The process of claim 1 wherein said alkali metal aromatic carboxylateis potassium benzoate.

6. The process of claim 1 wherein said adjuvant is melamine.

References Cited UNITED STATES PATENTS 2,794,830 6/1957 Raecke et a1.260-515 2,823,230 2/ 1958 Raecke et al 260-515 3,023,234 2/1962 Schuttet a1 -515 JAMES A. PATTEN, Primary Examiner I. GARNER, AssistantExaminer

